A. Field of the Invention
The present invention relates to a shelf in a relay rack for mounting multiple circuit boards in individual EMI-suppressing card slots. More specifically, the present invention relates to a shelf in a relay rack for mounting a power card, a filter card, a hub card, and multiple modem cards in individual EMI-suppressing card slots.
B. Description of the Prior Art
Computer network traffic on LANs and WANs passes at a variety of data transfer rates and through a wide array of equipment including hubs, switches, routers, servers, modems, and filters. A variety of cabling is used to support such traffic, including twisted pair cable, coaxial cable, and fiber optics cable. Each type of cable has advantages and disadvantages.
The primary advantage of twisted pair is that it is inexpensive to install and it is already widely installed. Virtually all homes and businesses in the United States have access to plain old telephone service (xe2x80x9cPOTSxe2x80x9d), which uses twisted pair. However, current commercially available modem technology using POTS permits only a relatively low data transfer rate of 56 Kbps. An emerging commercially available technology, asymmetric digital subscriber line (xe2x80x9cADSLxe2x80x9d), also uses POTS and could speed downstream (into the user""s computer) data transfer rates to as high as 8 Mbps, with 1.5 Mbps a more likely average transfer rate. However, even the downstream data transfer rate possible with ADSL may not be high enough to satisfy demand. And the upstream (leaving the user""s computer) data transfer rate for commercially available ADSL is LAW OFFICES it currently limited to 56 Kbps. Additionally, ADSL has been found to be affected by noise from adjacent circuitry. When noise is detected in an ADSL datastream, the transfer rate is automatically decreased in order to attempt to eliminate the effect of the noise.
Cable modems can support downstream data transfer rates at higher speeds of up to 30 Mbps. However, such high-speed service is only available to users with access to a coaxial cable connection, typically a cable television network. Such connections exist in far fewer homes and businesses in the United States than POTS. Moreover, current cable modems also suffer from a relatively slow, 56 Kbps, upstream data transfer rate.
Optical fiber can support data transfer rates of at least 500 Mbps and can transmit signals over the longest distances without repeaters. Fiber-based network segments can be extended 20 times farther than the copper segments of POTS. Additionally, fiber is immune to EMI, which reduces the number of retransmissions required. However, very few homes and businesses have access to a fiber-only network, and installation of a fiber-only network likely will require billions of dollars and many years to complete.
Because POTS is so widely available today, adaptation of a twisted-pair network for high-speed data transfers of 10-100 Mbps is highly desirable. However, current attempts at such adaptation are limited to the relatively short, 100-meter runs of cable in a typical Ethernet network. Current Ethernet connections cannot be extended more than about 1 kilometer. EMI essentially overwhelms the data signal transmitted over twisted-pair lines in cable runs longer than these relatively short distances. Primary sources of EMI on twisted-pair cables are the points at which cable is physically located near noise-emitting electronic equipment, like a modem shelf.
Where cable runs converge in a modem shelf, the modem circuitry and the cables themselves all act as broadcast antennae that generate EMI, which of course, is received by neighboring circuitry and cabling acting as receiving antennae. Current modem shelves do little to suppress EMI because, at today""s 56-Kbps data rates, EMI does not create problems. Essentially, EMI is typically at a high enough frequency (when compared to a 56-Kbps signal) that the EMI does not unduly degrade the shape of the signal. In the Ethernet environment, EMI is addressed by keeping cable runs short and shielding cable and equipment. Modems, hubs, routers, power cards and other devices on circuit cards are all mounted side by side in typical modem shelves. The more densely the cards are packed in a shelf, the greater the number of physical lines that can be installed within a given space.
To support higher data transfer rates over the twisted-pair cabling of POTS, EMI generated in modem shelves must be minimized. One method of suppressing EMI is to surround electrical circuitry or other EMI-generating devices with a grounded, metallic box, commonly called a Faraday cage. Conventional modem shelves act to some degree as Faraday cages for the entire group of cards installed in them. However, such conventional shelves have no structure or other mechanism for isolating individual cards from EMI generated by neighboring cards. Additionally, EMI originating outside the shelf creeps into the interior of the shelf through the front and back of the shelf. Conventional modem shelves thus do not provide sufficient EMI suppression to enable high-speed data communications using POTS.
A modem shelf according to the present invention fits within standard racks used to mount circuit cards. The shelf comprises top, bottom, side and back plates made of aluminum, but any EMI-suppressing material is appropriate. These plates suppress EMI that might enter the shelf from the outside and EMI that might leave the shelf and interfere with operation of cards in adjacent shelves or xe2x80x9cpollutexe2x80x9d signals traveling through nearby cabling. Dividers, also made of aluminum or other EMI-suppressing material, separate the interior of the shelf into individual card slots and suppress EMI that would otherwise pass between cards. Individual front plates suppress EMI entering or leaving through the front end of the shelf. These front plates may or may not be part of the circuit card assemblies, but they are part of the card assemblies in the preferred embodiment of the invention. Thus, each card slot provides an environment isolating cards in the shelf from surrounding EMI and suppressing EMI generated by the cards. The shelf""s EMI-isolation and -suppression characteristics enable the circuit cards mounted in the shelf to perform without EMI-induced problems and preserves the integrity of high-speed data streams carried through nearby twisted pair cables and electronic circuitry.
The invention, as embodied and broadly described below, is a shelf for mounting a plurality of circuit cards, comprising: an exterior shell made of EMI-shielding material and defining an internal space, comprising: a top, a first side connected to the top, a second side connected to the top, and a bottom connected to the first side and the second side; and a divider made of EMI-shielding material that: is connected to the top of the exterior shell and the bottom of the exterior shell and separates the internal space into a first card slot and a second card slot; such that the portion of the exterior shell surrounding each of the first and second card slots and the divider suppresses EMI generated by one of the circuit cards mounted in either the first or the second card slot.
The shelf may also comprise divider plates, attached to the card assemblies in the preferred embodiment, which further suppress EMI emitted in a direction perpendicular to the circuit cards. The front plates on each card assembly may be bent inward toward the interior of the shelf so that the bent portion overlaps with the front edge of one of the dividers connected to the top and bottom of the shelf. This overlap xe2x80x9csealsxe2x80x9d the front side of the card slot and prevents EMI from leaving through the front plate. The bent portion of the front plates and the front edges of the dividers may also include stamped protrusions with apertures extending through the protrusions, which further suppress EMI emitted toward the front plate. The mounting brackets attached to the sides of the shelf may be configured to be attached in multiple locations and orientations so that the same bracket can be used to mount the shelf in a variety of relay racks. Holes may extend through the top and bottom of the shelf to aid in cooling the circuitry on the cards mounted in the shelf. The holes in the top are small enough to prevent debris (e.g., a #4 screw) from falling into the interior of the shelf and the holes in the bottom are large enough to allow debris to fall out of the shelf.